Thermite reactivity with ball milled aluminum-zirconium fuel particles

Thermite reactivity is a function of the reactant particle size. However, metallurgical strategies that purposefully engineer larger fuel particles to be as reactive as their smaller scale counterparts could be a transformative development in thermite combustion. In this study, ball milled aluminum/zirconium (Al:Zr) particles are prepared with four different Al:Zr particle size ranges (0–10, 10–32, 32–53 and 53–75 µm) and similar internal microstructures to understand the influence of particle size on reactivity within thermite mixtures. The thermites are prepared by combining the Al:Zr particles with molybdenum trioxide (MoO3) particles and reactivity was assessed using flame speed measurements. The results showed that for the smallest size range (0–10 µm) and the largest size range (53–75 µm), flame speeds were ∼ 4 cm/s while the two middle Al:Zr size ranges had flame speeds of only ∼ 2 cm/s. The unexpected size dependence is attributed to a change in the thermal transport mechanism controlling flame propagation, from thermal conduction at small Al:Zr powder sizes to thermal convection at large sizes. The rate of energy release during the thermite reaction also plays a role in controlling the flame speed. We propose that the faster flame speed for the 0–10 µm Al:Zr particles result from more contact points between the fuel and oxidizer powders, leading to improved oxygen mass diffusion and greater energy release promoting thermal transport by conduction. In contrast, the faster speed for the largest 53–75 µm Al:Zr particles is attributed to increased thermal transport via convection due to larger pore sizes. The 10–32 and 32–53 µm Al:Zr particle sizes are either too big or too small for reactivity or thermal transport to be optimized, resulting in slightly slower flame speeds. These results show that larger Al:Zr particles can be designed to exhibit reactivity representative of smaller size particles by utilizing composite particles that leverage intermetallic reactions while also exploiting multiple modes of heat transfer.